Biuret modified isocyanate compositions have long been known in the polyurethane industry. German Pat. No. 1,101,394 contains examples in which biuret modified isocyanates are prepared via partial saponification (with H.sub.2 O or H.sub.2 S) of isocyanate while heating. The biuret structures are formed in a two step reaction sequence. The initial saponification reaction leads to the formation of urea linkages. Under the influence of heat these ureas will further react to form biuret structures: ##STR1## plus higher order biuret structures.
This technique is effective because step A is a relatively slow reaction, such that the intermediate urea is consumed (in step B) at a rate which is comparable to its rate of formation. Moreover the active hydrogen species (H.sub.2 O or H.sub.2 S) has sufficient time to diffuse throughout the isocyanate before reacting, such that the reaction proceeds evenly throughout the bulk of the sample.
Biuret modified isocyanates may also be prepared via the reaction of mono or polyamines with polyisocyanates at sufficiently elevated temperatures. Here also, the reaction sequence involves two steps: ##STR2## plus higher order biuret structures.
This technique is generally not satisfactory because step C is a veyy fast reaction, such that the starting amine is unable to diffuse into the bulk of the isocyanate before reacting. Thus the bulk of the reaction in step C occurs at the point of mixing between the amine and the isocyanate and produces a very high concentration of urea intermediate at the point of mixing. In the vast majority of cases this concentration effect results in the formation of solid precipitates. These urea precipitates must be redissolved before biuret formation (step D) can occur. This resolubilization can be accomplished by heating and vigorous agitation, but the temperatures required are generally rather extreme (usually &gt;150.degree. C.), and much higher than is needed to promote biuretization (step D). The use of such high temperatures is uneconomical and can result in undesirable side reactions. These side reactions can lead to product discoloration and, in some cases, to the liberation of volatile and toxic by-products (via a series of biuret exchange reactions).
Several methods have been developed to overcome the problems of solids formation, discussed above. By far the oldest and most straightforward method for avoiding the formation of urea precipitates is to conduct the entire reaction in an appropriate solvent. Generally, however, the use of solvents is impractical for economic and environmental reasons. According to the teachings of U.S. Pat. No. 4,147,714, it is possible to prepare biuret modified isocyanate compositions by reacting a liquid isocyanate with an amine vapor. The use of a vapor greatly increases the surface area of the reaction interface between the amine and isocyanate--thereby eliminating high local concentrations of insoluble urea intermediates. Similarly, the precipitation of insoluble polyurea intermediates can be eliminated, according to U.S. Pat. No. 3,824,266, by using amines which are slow-reacting (i.e., sterically or electronically deactivated). Presumably this "deactivation" permits the amine to diffuse into the bulk isocyanate before reacting--thereby increasing the size of the reaction interface. Finally, U.S. Pat. No. 3,441,588 teaches that biuret prepolymers may be conveniently prepared, without the undesirable formation of solid urea precipitates, by employing a high molecular weight polyether diamine in the reaction with isocyanates. It appears likely that the long polyether chain acts as an internal "solvent" for the intermediate urea, thereby inhibiting phase separation. In spite of the "solvent" effect of the polyether chain, it is generally not possible to prepare analogous biuret modified isocyanate compositions from polyether polyamines having reactive amine functionalities of greater than 2 (i.e., polyether triamines), without using inert solvents or extreme reaction conditions (i.e., temperatures of much greater than 100.degree. C). These fast-reacting high functionality amines can react with the isocyanate, crosslink, and gel before diffusing into the bulk of the isocyanate sample. This problem is most severe in the more reactive (aromatic) polyisocyanates, such as MDI. Gelation and separation problems may also be encountered when attempting to react polyether diamines with isocyanates having functionality greater than 2.
Much of the interest in biurets, as modifying additives for isocyanates, stems from the poor solubility of the corresponding urea systems. Biuretization substantially increases solubility--perhaps by interfering with the formation of hydrogen bond networks and oligomers. Whereas all of the patent documents cited thus far pertain specifically to biuret modified isocyanate compositions, U.S. Pat. No. 3,943,158 pertains to urea modified compositions. These urea prepolymers are formed by the reaction of diisocyanates with certain bis-secondary diamines. The use of secondary amines undoubtedly interferes with the formation of hydrogen bond networks in a manner which is closely analogous to that of biuretization.